An often used procedure of the chemical or physical alteration of material involves the passage of such material through a bed of adsorbent, catalyst, etc. depending on the intended result. When a continuous process is employed, a packed bed presents certain problems. While is is desired to have short cycle times in order to minimize bed inventory and equipment size, longer cycle times are generally more energy efficient. Furthermore, there is a need for an elaborate and expensive valve assembly for cycling various streams among various beds, which can lead to increased maintenance costs. These operating problems become more acute as one tries to minimize capital expense by reducing cycle times. Those skilled in the art have long recognized the advantages of moving and fluidized beds over packed beds. Such advantages include a greater energy efficiency because of the reduction of cyclic and transient energy losses and because of higher product recoveries.
A problem with the use of certain adsorbent particles in adsorbent beds is attrition of the particle by abrasion of its surface. In fixed beds, fluid flow may cause adjacent particles to contact and abrade each other, especially when a localized portion or the entire bed is accidentally fluidized. Movement may also be caused by external forces such as vibrations due to nearby compressor or location of the bed on a moving vehicle. In moving and fluidized beds, the problem of attrition is magnified. Excessive particle attrition is caused, for example, by abrasion among bed particles, abrasion with bed walls and bed internals and distributor jet impingement and abrasion in circulation conduits to and from the bed. High particle attrition contributes to product contamination, particle loss, plugging of downstream equipment, high filtration costs, and unstable fluidization behaviour such as channeling, slugging or increased entrainment.
The problem of particle attrition is especially severe with high porosity bed particles such as molecular sieves. Molecular sieve beads or pellets consist essentially of zeolite crystals and a clay binder material. Due to the ceramic nature of both these materials, the surface is highly abrasive and subject to attrition. The amount of surface attrition caused by an impact on the particle depends on the particle's momentum, which is the product of its mass and its velocity. Therefore, smaller particles traveling at low speeds, i.e. in a bed having low fluidization velocity, do not suffer as much attrition as large, highly fluidized particles. The total amount of attrition, as measured by the amount of dust generated in the adsorbent bed, includes surface attrition and attrition due to the breaking up of the entire particle.
It has been possible to employ molecular sieves in fixed beds without excessive attrition. However, it has not generally been economical to use molecular sieves for moving beds, except in very specialized applications such as cracking petroleum fractions for gasoline. In this approach, very small molecular sieve particles made up of about 80% clay binder are formed. Molecular sieves employed in packed beds generally have only about 20% clay. The particles containing mostly clay have a higher crush strength, but a lower mass transfer efficiency, than the conventional molecular sieve particles. In catalytic cracking, the granular type particles are swept along by a carrier gas with which they react. The attrition experienced is somewhat less than that of particles containing 20% clay, and is nearly equal to the amount of sieve that must be replaced anyway due to loss of reactivity, so the attrition does not prevent the use of these particles in a moving bed. The slight reduction in total attrition is believed to be largely attributable to fewer particles becoming pulverized, as opposed to any significant difference in surface attrition of the "harder" particles.
Thus, the capability to employ molecular sieves in moving and fluidized beds while keeping attrition low without a significant reduction in efficiency would be highly desirable.
It is therefore an object of this invention to provide an attrition resistant molecular sieve.
It is another object of this invention to provide molecular sieves which can be employed in moving and fluidized beds while exhibiting low attrition and retaining high mass transfer or reactive capabilities.
It is another object of this invention to provide a method of treating generally spherical, high porosity, friable molecular sieve particles such that they can be employed in moving and fluidized beds while exhibiting low attrition and retaining high mass transfer or reactive capabilities.